The 3rd Generation Partnership Project (3GPP) is responsible for the standardization of the UMTS (Universal Mobile Telecommunication Service) system, and LTE (Long term Evolution) is now under discussion as a next generation mobile communication system of the UMTS system. LTE is a technology for realizing high-speed packet-based communication that can reach a data rates of about 100 Mbps on the downlink and about 50 Mbps on the uplink. To this end, schemes and mechanisms are under discussions, for example, a scheme to reduce the number of network nodes in conventional UMTS networks. As an example, the base station in LTE, also known as eNB (enhanced Node B), will perform the functions of a conventional radio access network (RNC) node and of a UMTS Node B. In addition, eNBs in LTE will interact directly with the core network and with other eNBs
Another ongoing work on the LTE network (i.e. on the UMTS Terrestrial Radio Access Network LTE (UTRAN-LTE), is a mechanism known as discontinuous reception (DRX) which is defined to save battery time and resources of user equipments (UE). With DRX, a UE can turn on and off reception of layer 1/layer 2 (L1/L2) control in radio resource control connected state or connected mode (RRC_connected). In order to save battery time, the connected mode UE, while being in sleep mode during a predetermined DRX cycle period, wakes up at specific timings in order to check/monitor for possible control channels allocated by the eNB to determine if there is data to receive. When there is no data to receive, the UE switches to the sleep mode and keeps the sleep mode until the next wake-up time. The control channel checked/monitored by the UE is known as PDCCH (Physical Downlink Control Channel). When there is data to receive, the UE receives the data from the eNB and sends a response signal (ACK/NACK) indicating a successful or a failure in the reception of the data transmitted based on a protocol known as HARQ (Hybrid Automatic Repeat Request) protocol. In the technical specification 3GPP TS 36.321 entitled: “Evolved Universal Terrestrial Radio Access (E-UTRA) Medium Access Control (MAC) protocol specification”, the different timings/timers related to DRX, are defined. As an example, DRX uses one or two predefined cycles (long and/or short cycles) at the beginning of which the UE should monitor the PDCCH over a certain amount of TTIs (Transmission Time Interval) under a so called Active Time. During the Active Time, the UE monitors the PDCCH for PDCCH-subframe(s). The number of consecutive PDCCH-subframe(s) at the beginning of the DRX cycle (i.e. during the Active Time) is known as the “On-duration Timer”. The On-duration timer in the beginning of each cycle also defines how long a UE should monitor the PDCCH. The PDCCH can carry both downlink assignments as well as uplink grants scheduled by eNB.
Whether the UE is awake (i.e. monitors the PDCCH) or is asleep after the On-duration period depends on activity, i.e. possible receptions of PDCCH control data during that period. In order to avoid unnecessary scheduling and to avoid wasting of radio resources, the eNB should know the state of the UE when transmitting downlink data to the UE. In other words, the eNB should know if the UE is in DRX or not. Therefore, it is defined in the above mentioned specification 3GPP TS 36.321 a set of rules for changing from the active state to DRX and back. An example of such a rule is where the UE fails in decoding the PDCCH successfully during the On-duration timer, i.e. there are no control data. In such an event, the UE enters the sleep mode i.e. from an active state to DRX state. If on the other hand, the PDCCH indicates DL transmission, the UE starts or restarts a so called DRX inactivity timer. It should also start to use the short cycle if it is configured and when the short cycle timer expires the use to the short cycle is stopped. It should also be mentioned that it is also defined in the specification that if a so called HARQ RTT (round trip time) timer stops in a TTI, the UE should start a retransmission timer.
Because the eNB and the UE generally have similar timers and knowledge of the beginning times of the cycles (long), the state (active, DRX) of the UE does not have to be signaled explicitly between the eNB and the UE. Therefore, as long as the eNB and the UE are state synchronized with each other, the conventional DRX mechanism works well. In other words, if there are no errors in the control channels i.e. if the UE always can decode the PDCCH when there is a DL assignment or an UL grant, the inactivity timers of the UE and of the eNB run in the same phase, and the UE continues monitoring of the PDCCH even if the On-duration timer expired. However, if there is an error in PDCCH reception, the eNB starts the inactivity timer while the UE does not and the state synchronization between the UE and the eNB is thus lost. Therefore, in this case, the UE enters DRX state, whereas the eNB assumes that UE is still awake and that the UE continues to monitor the PDCCH for another time period equal to at least the inactivity timer. Furthermore, the eNB can continue to transmit PDCCH control information and to assign transmission resources for DL data and/or grants for UL data, during the entire long cycle (which can be in the range of 200 ms or even more) by pipelining inactivity timers and retransmission timers, while the UE does not monitor any of aimed assigned resources. This will lead to a waste of radio resources that instead can be allocated for data transmissions for others UEs (or users). In addition, unnecessary radio transmissions produce interference in the cells of the network, e.g. in one or several neighbouring cells to the cell where the UE is currently located. Furthermore, if the short cycle is configured, the UE that is currently operating in the long cycle and that has missed the control signalling (PDCCH) due to error in PDCCH reception, will not switch to the short cycle as expected by the base station (or eNB).